Marine pipelines for conveying hydrocarbons, generally being gas and/or crude oil, are typically laid from specialised pipe-laying vessels or barges. Marine pipelines include rigid pipes and flexible pipes, as defined in the standards published by the American petroleum institute (API) API 17J and API RP 17B. The rigid pipelines are deployed and lowered into the sea and down to the seabed in a controlled fashion, with the pipelines either being assembled from individual pipe sections on the vessel (referred to as “stovepiping”), or whilst spooled from one or more reels on the vessel (referred to as “reel pipelaying”). The flexible pipelines are deployed and lowered into the sea and down to the seabed whilst stored from a basket.
Part of the deployment of the pipeline from the vessel can involve passing the pipeline through one or more ‘launch means’ or ‘deployment means’. This can include apparatus or assemblies for straightening the pipeline as it leaves a reel, and to ensure that the pipeline is properly guided during launch, generally along a correct ‘launch trajectory’ or ‘laying path’, for delivering the pipeline from the vessel into the sea and down to the seabed.
Commonly, the pipelaying vessel includes a tower, (which may also be moveable, and particularly rotatable to an angle to the vertical). One common tower is a kind of derrick, with two fixed legs embedded in or anchored to the deck of the vessel. The tower is generally for supporting one or more tensioning devices arranged inside the frame of the derrick and able to tension the pipeline during the pipe-lay operation so as to support the weight of the pipeline in the water prior to reaching the seabed. This configuration helps to reinforce the stiffness of the tensioning arrangement. Some tensioning devices include ‘tracks’, often in the form of caterpillar tracks, able to allow or induce movement of the pipeline through the tensioning device whilst still applying sufficient tension thereto.
Referring now to the rigid pipelay system, it also can be desired to ‘hold’ a pipeline during a pipe-laying operation, generally using a ‘hang-off clamp’, to allow the attachment of accessories to the rigid pipeline, such as at one end, or to one or more intermediate locations along the pipeline. Such accessories can be relatively large and heavy, and are commonly designed to be welded to or into the pipeline. Examples of such accessories are initiation fittings, pipeline end terminations (PLETs), pipe valves, and tee assemblies.
To allow access to the rigid pipeline to attach such accessories, one idea is to move the pipeline away from its launch trajectory or laying path. For example, WO03067019A shows the movement of a hang-off clamp and a launch pipeline together along a rail for connection to a ‘heavy device’. Once the heavy device is connected to the pipeline, the assembly is returned to the launch position next to the tower.
Another idea is to maintain the position of the pipeline in its launch trajectory, generally supported by a hang-off clamp, and move the tensioning assemblies out of the launch trajectory or firing line. EP2005050B shows a tower able to move a tensioner frame between an active position and a retracted and non-operable position spaced from the pipeline launch trajectory.
WO2008/007945A shows a tensioner frame which is openable by having two pivotal frame parts to allow the introduction/removal of a pipeline, whilst requiring a parallel linkage bar system to allow each of its multiple track units to be mobile with a frame.
Referring now to the flexible pipe laying system, it also can be desired to pass flexible end fittings down through the firing line from above all the tensioning devices in the stack.
In both systems, space is limited within the tower or derrick, whose legs are generally only a few meters apart, such as 10 m. However, as exploration and production goes to greater depths, such that longer pipelines and stronger pipelines are required, there is a need to deploy heavier and larger subsea accessories and flexible end fittings for and in such pipelines. The practice of any conventional openable tensioning frame in a tower/derrick frame known to date provides a maximum clearance from a closed position of 1.8 m to an open position of 2.8 m only, i.e. only 1 extra meter within the leg structure, which thereby limits some of the now desired accessory installation.